How to rotate all the keyframes of root bone?
I think the easiest way to do this - it just rotate all armature as object. To do this: 


Make sure, that you select armature in object mode. If you are in Pose mode, switch to object.
Then rotate object as you need.


Also, you can add new root bone, and parent "old root bone" to the new one. In this case you can use it to position and rotate all bones in pose mode.